In computing, a process is the instance of a computer program, being executed. It contains its activity. Depending on the operating system, a process may be made up of multiple threads of execution that execute instructions concurrently. While a computer program is a passive collection of instructions, a process is the actual execution of those instructions. Several processes may be associated with the same program. Multitasking is a method to allow multiple processes to other system resources; each CPU executes a single task at a time. However, multitasking allows each processor to switch between tasks that are being executed without having to wait for each task to finish. Depending on the operating system implementation, switches could be performed when tasks perform input/output operations, when a task indicates that it can be switched, or on hardware interrupts. A common form of multitasking is time-sharing. Time-sharing is a method to allow high responsiveness for interactive user applications.
In time-sharing systems, context switches are performed which makes it seem like multiple processes are being executed on the same processor. This seeming execution of multiple processes is called concurrency. For security and reliability, most modern operating systems prevent direct communication between independent processes, providing mediated and controlled inter-process communication functionality. In general, a computer system process consists of the following resources: An image of the executable machine code associated with a program. Memory. Operating system descriptors of resources that are allocated to the process, such as file descriptors or handles, data sources and sinks. Security attributes, such as the process' set of permissions. Processor state, such as the content of registers and physical memory addressing; the state is stored in computer registers when the process is executing, in memory otherwise. The operating system holds most of this information about active processes in data structures called process control blocks.
Any subset of the resources at least the processor state, may be associated with each of the process' threads in operating systems that support threads or child processes. The operating system keeps its processes separate and allocates the resources they need, so that they are less to interfere with each other and cause system failures; the operating system may provide mechanisms for inter-process communication to enable processes to interact in safe and predictable ways. A multitasking operating system may just switch between processes to give the appearance of many processes executing though in fact only one process can be executing at any one time on a single CPU, it is usual to associate a single process with a main program, child processes with any spin-off, parallel processes, which behave like asynchronous subroutines. A process is said to own resources. However, in multiprocessing systems many processes may run off of, or share, the same reentrant program at the same location in memory, but each process is said to own its own image of the program.
Processes are called "tasks" in embedded operating systems. The sense of "process" is "something that takes up time", as opposed to "memory", "something that takes up space"; the above description applies to both processes managed by an operating system, processes as defined by process calculi. If a process requests something for which it must wait, it will be blocked; when the process is in the blocked state, it is eligible for swapping to disk, but this is transparent in a virtual memory system, where regions of a process's memory may be on disk and not in main memory at any time. Note that portions of active processes/tasks are eligible for swapping to disk, if the portions have not been used recently. Not all parts of an executing program and its data have to be in physical memory for the associated process to be active. An operating system kernel that allows multitasking needs processes to have certain states. Names for these states are not standardised. First, the process is "created" by being loaded from a secondary storage device into main memory.
After that the process scheduler assigns it the "waiting" state. While the process is "waiting", it waits for the scheduler to do a so-called context switch and load the process into the processor; the process state becomes "running", the processor executes the process instructions. If a process needs to wait for a resource, it is assigned the "blocked" state; the process state is changed back to "waiting". Once the process finishes execution, or is terminated by the operating system, it is no longer needed; the process is removed or is moved to the "terminated" state. When removed, it just waits to be removed from main
Write (system call)
The write is one of the most basic routines provided by a Unix-like operating system kernel. It writes data from a buffer declared by the user to a given device, maybe a file; this is the primary way to output data from a program by directly using a system call. The destination is identified by a numeric code; the data to be written, for instance a piece of text, is defined by a pointer and a size, given in number of bytes. Write thus takes three arguments: The file code; the pointer to a buffer where the data is stored. The number of bytes to write from the buffer; the write call interface is standardized by the POSIX specification. Data is written to a file by calling the write function; the function prototype is: In above syntax, ssize_t is a typedef. It is a signed data type defined in stddef.h. Note that write does not return an unsigned value; the write function returns the number of bytes written into the array, which may at times be less than the specified nbytes. It returns -1 if an exceptional condition is encountered.
Listed below are some errors. The errors are macros listed in errno.h. The write system call is not an ordinary function, in spite of the close resemblance. For example, in Linux with the x86 architecture, the system call uses the instruction INT 80H, in order to transfer control over to the kernel; the write system call, its counterpart read, being low level functions, are only capable of understanding bytes. Write cannot be used to write records, like classes. Thus, higher level input-output functions are required; the high-level interface is preferred, as compared to the cluttered low-level interface. These functions call other functions internally, these in turn can make calls to write, giving rise to a layered assembly of functions. With the use of this assembly the higher level functions can collect bytes of data and write the required data into a file. Fwrite getchar fprintf pwrite read sync POSIX write C_Programming/C_Reference/stdio.h/fwrite at Wikibooks
A computer file is a computer resource for recording data discretely in a computer storage device. Just as words can be written to paper, so can information be written to a computer file. Files can be transferred through the internet. There are different types of computer files, designed for different purposes. A file may be designed to store a picture, a written message, a video, a computer program, or a wide variety of other kinds of data; some types of files can store several types of information at once. By using computer programs, a person can open, change and close a computer file. Computer files may be reopened and copied an arbitrary number of times. Files are organised in a file system, which keeps track of where the files are located on disk and enables user access; the word "file" derives from the Latin filum."File" was used in the context of computer storage as early as January 1940. In Punched Card Methods in Scientific Computation, W. J. Eckert stated, "The first extensive use of the early Hollerith Tabulator in astronomy was made by Comrie.
He used it for building a table from successive differences, for adding large numbers of harmonic terms". "Tables of functions are constructed from their differences with great efficiency, either as printed tables or as a file of punched cards." In February 1950: In a Radio Corporation of America advertisement in Popular Science Magazine describing a new "memory" vacuum tube it had developed, RCA stated: "the results of countless computations can be kept'on file' and taken out again. Such a'file' now exists in a'memory' tube developed at RCA Laboratories. Electronically it retains figures fed into calculating machines, holds them in storage while it memorizes new ones - speeds intelligent solutions through mazes of mathematics." In 1952, "file" denoted, information stored on punched cards. In early use, the underlying hardware, rather than the contents stored on it, was denominated a "file". For example, the IBM 350 disk drives were denominated "disk files"; the introduction, circa 1961, by the Burroughs MCP and the MIT Compatible Time-Sharing System of the concept of a "file system" that managed several virtual "files" on one storage device is the origin of the contemporary denotation of the word.
Although the contemporary "register file" demonstrates the early concept of files, its use has decreased. On most modern operating systems, files are organized into one-dimensional arrays of bytes; the format of a file is defined by its content since a file is a container for data, although, on some platforms the format is indicated by its filename extension, specifying the rules for how the bytes must be organized and interpreted meaningfully. For example, the bytes of a plain text file are associated with either ASCII or UTF-8 characters, while the bytes of image and audio files are interpreted otherwise. Most file types allocate a few bytes for metadata, which allows a file to carry some basic information about itself; some file systems can store arbitrary file-specific data outside of the file format, but linked to the file, for example extended attributes or forks. On other file systems this can be done via software-specific databases. All those methods, are more susceptible to loss of metadata than are container and archive file formats.
At any instant in time, a file might have a size expressed as number of bytes, that indicates how much storage is associated with the file. In most modern operating systems the size can be any non-negative whole number of bytes up to a system limit. Many older operating systems kept track only of the number of blocks or tracks occupied by a file on a physical storage device. In such systems, software employed other methods to track the exact byte count; the general definition of a file does not require that its size have any real meaning, unless the data within the file happens to correspond to data within a pool of persistent storage. A special case is a zero byte file. For example, the file to which the link /bin/ls points in a typical Unix-like system has a defined size that changes. Compare this with /dev/null, a file, but its size may be obscure. Information in a computer file can consist of smaller packets of information that are individually different but share some common traits. For example, a payroll file might contain information concerning all the employees in a company and their payroll details.
A text file may contain lines of corresponding to printed lines on a piece of paper. Alternatively, a file may contain an arbitrary binary image or it may contain an executable; the way information is grouped into a file is up to how it is designed. This has led to a plethora of more or less standardized file structures for all imaginable purposes, from the simplest to the most complex. Most computer files are used by computer programs which create, modify or delete the files for their own use on an as-needed basis; the programmers who create the programs decide what files are needed, how they are to be used and their names. In some cases, computer pr
A filename is a name used to uniquely identify a computer file stored in a file system. Different file systems impose different restrictions on filename lengths and the allowed characters within filenames. A filename may include one or more of these components: host – network device that contains the file device – hardware device or drive directory – directory tree file – base name of the file type – indicates the content type of the file version – revision or generation number of the fileThe components required to identify a file varies across operating systems, as does the syntax and format for a valid filename. Discussions of filenames are complicated by a lack of standardization of the term. Sometimes "filename" is used to mean the entire name, such as the Windows name c:\directory\myfile.txt. Sometimes, it will be used to refer to the components, so the filename in this case would be myfile.txt. Sometimes, it is a reference that excludes an extension, so the filename would be just myfile.
Around 1962, the Compatible Time-Sharing System introduced the concept of a file. Around this same time appeared the dot as a filename extension separator, the limit to three letter extensions might have come from 16-bit RAD50 character encoding limits. Traditionally, most operating system supported filenames with only uppercase alphanumeric characters, but as time progressed, the number of characters allowed increased; this led to compatibility problems. In 1985, RFC 959 defined a pathname to be the character string that must be entered into a file system by a user in order to identify a file. Around 1995, VFAT, an extension to the MS-DOS FAT filesystem, was introduced in Windows 95 and Windows NT, it allowed mixed-case Unicode long filenames, in addition to classic "8.3" names. One issue was migration to Unicode. For this purpose, several software companies provided software for migrating filenames to the new Unicode encoding. Microsoft provided migration transparent for the user throughout the vfat technology Apple provided "File Name Encoding Repair Utility v1.0".
The Linux community provided “convmv”. Mac OS X 10.3 marked Apple's adoption of Unicode 3.2 character decomposition, superseding the Unicode 2.1 decomposition used previously. This change caused problems for developers writing software for Mac OS X. An absolute reference includes all directory levels. In some systems, a filename reference that does not include the complete directory path defaults to the current working directory; this is a relative reference. One advantage of using a relative reference in program configuration files or scripts is that different instances of the script or program can use different files; this makes an relative path composed of a sequence of filenames. Unix-like file systems allow a file to have more than one name. Windows supports hard links on NTFS file systems, provides the command fsutil in Windows XP, mklink in versions, for creating them. Hard links are different from classic Mac OS/macOS aliases, or symbolic links; the introduction of LFNs with VFAT allowed filename aliases.
For example, longfi~1.??? with a maximum of eight plus three characters was a filename alias of "long file name.???" as a way to conform to 8.3 limitations for older programs. This property was used by the move command algorithm that first creates a second filename and only removes the first filename. Other filesystems, by design, provide only one filename per file, which guarantees that alteration of one filename's file does not alter the other filename's file; some filesystems restrict the length of filenames. In some cases, these lengths apply to the entire file name, as in 44 characters on IBM S/370. In other cases, the length limits may apply to particular portions of the filename, such as the name of a file in a directory, or a directory name. For example, 9, 11, 14, 21, 31, 30, 15, 44, or 255 characters or bytes. Length limits result from assigning fixed space in a filesystem to storing components of names, so increasing limits requires an incompatible change, as well as reserving more space.
A particular issue with filesystems that store information in nested directories is that it may be possible to create a file with a complete pathname that exceeds implementation limits, since length checking may apply only to individual parts of the name rather than the entire name. Many Windows applications are limited to a MAX_PATH value of 260, but Windows file names can exceed this limit. Many file systems, including FAT, NTFS, VMS systems, allow a filename extension that consists of one or more characters following the last period in the filename, dividing the filename into two parts: a base name or stem and an extension or suffix used by some applications to indicate the file type. Multiple output files created by an application use various extensions. For example, a compiler might use the extension FOR for source input file, OBJ for the object output and LST for the listing. Although there are some common extensions, they are arbitrary and a different application might use REL and RPT.
On filesystems that do not segregate the extension, files will have a longer extension such as html. There is no general encoding standard for filenames
A computer program is a collection of instructions that performs a specific task when executed by a computer. A computer requires programs to function. A computer program is written by a computer programmer in a programming language. From the program in its human-readable form of source code, a compiler can derive machine code—a form consisting of instructions that the computer can directly execute. Alternatively, a computer program may be executed with the aid of an interpreter. A collection of computer programs and related data are referred to as software. Computer programs may be categorized along functional lines, such as application software and system software; the underlying method used for some calculation or manipulation is known as an algorithm. The earliest programmable machines preceded the invention of the digital computer. In 1801, Joseph-Marie Jacquard devised a loom that would weave a pattern by following a series of perforated cards. Patterns could be repeated by arranging the cards.
In 1837, Charles Babbage was inspired by Jacquard's loom to attempt to build the Analytical Engine. The names of the components of the calculating device were borrowed from the textile industry. In the textile industry, yarn was brought from the store to be milled; the device would have had a "store"—memory to hold 1,000 numbers of 40 decimal digits each. Numbers from the "store" would have been transferred to the "mill", for processing, and a "thread" being the execution of programmed instructions by the device. It was programmed using two sets of perforated cards—one to direct the operation and the other for the input variables. However, after more than 17,000 pounds of the British government's money, the thousands of cogged wheels and gears never worked together. During a nine-month period in 1842–43, Ada Lovelace translated the memoir of Italian mathematician Luigi Menabrea; the memoir covered the Analytical Engine. The translation contained Note G which detailed a method for calculating Bernoulli numbers using the Analytical Engine.
This note is recognized by some historians as the world's first written computer program. In 1936, Alan Turing introduced the Universal Turing machine—a theoretical device that can model every computation that can be performed on a Turing complete computing machine, it is a finite-state machine. The machine can move the tape forth, changing its contents as it performs an algorithm; the machine starts in the initial state, goes through a sequence of steps, halts when it encounters the halt state. This machine is considered by some to be the origin of the stored-program computer—used by John von Neumann for the "Electronic Computing Instrument" that now bears the von Neumann architecture name; the Z3 computer, invented by Konrad Zuse in Germany, was a programmable computer. A digital computer uses electricity as the calculating component; the Z3 contained 2,400 relays to create the circuits. The circuits provided a floating-point, nine-instruction computer. Programming the Z3 was through a specially designed keyboard and punched tape.
The Electronic Numerical Integrator And Computer was a Turing complete, general-purpose computer that used 17,468 vacuum tubes to create the circuits. At its core, it was a series of Pascalines wired together, its 40 units weighed 30 tons, occupied 1,800 square feet, consumed $650 per hour in electricity when idle. It had 20 base-10 accumulators. Programming the ENIAC took up to two months. Three function tables needed to be rolled to fixed function panels. Function tables were connected to function panels using heavy black cables; each function table had 728 rotating knobs. Programming the ENIAC involved setting some of the 3,000 switches. Debugging a program took a week; the programmers of the ENIAC were women who were known collectively as the "ENIAC girls." The ENIAC featured parallel operations. Different sets of accumulators could work on different algorithms, it used punched card machines for input and output, it was controlled with a clock signal. It ran for eight years, calculating hydrogen bomb parameters, predicting weather patterns, producing firing tables to aim artillery guns.
The Manchester Baby was a stored-program computer. Programming transitioned away from setting dials. Only three bits of memory were available to store each instruction, so it was limited to eight instructions. 32 switches were available for programming. Computers manufactured; the computer program was written on paper for reference. An instruction was represented by a configuration of on/off settings. After setting the configuration, an execute button was pressed; this process was repeated. Computer programs were manually input via paper tape or punched cards. After the medium was loaded, the starting address was set via switches and the execute button pressed. In 1961, the Burroughs B5000 was built to be programmed in the ALGOL 60 language; the hardware featured circuits to ease the compile phase. In 1964, the IBM System/360 was a line of six computers each having the same instruction set architecture; the Model 30 was the least expensive. Customers could retain the same application software; each System/360 model featured multiprogramming.
With operating system support, multiple programs could be in memory at once. When one was waiting for input/output, another could compute; each model could emulate other computers. Customers could upgrade to the System/360 and ret